DEGRADATION OF FATTY ACIDS
FADH 2 Electron transfer
flavoprotein
2Cytb12+ + 2H+
2Cytb,3+
Electron transport chain
FAD.Enz
The oxidation catalyzed by acyl-CoA dehydrogenase is analogous to succinate dehydrogenation in the citric acid cycle; as in both the reaction:
(a) The enzyme is bound to the inner membrane.
(b) A double bond is introduced into a carboxylic acid between the α and β carbons.
(c) FAD is the electron acceptor, and
(d) Electrons from the reaction ultimately enter the respiratory chain and are carried to O2 with the concomitant synthesis of 2 ATP molecular per electron pair.
Second step : Hydration of α, β-unsaturated acyl-CoAs.
In this step, a mole of water is added to the double bond of the trans-∆2-enoyl-CoA to form the L-stereoisomer of β-hydroxyacyl-CoA (also called 3-hydroxyacyl-CoA). The reaction is catalyzed by enoyl-CoA hydratase or crotonase and has broad specificity with respect to the length of the acyl group. However, its activity decreases progressively with increasing chain length of the substrate (it may be noted that the enzyme will also hydrate α, β-cis unsaturated acyl-CoA, but in this case D (–)-β-hydroxyacyl-CoA is found).
R–CH2–CH=CH–C–S–CoA + H2O β α O
trans ∆ G´ = –0.75 kcal/mole
R–CH2–CH–CH–C–S–CoA H O
L(+)–β–hydroxyacyl-CoAOH
This reaction catalyzed by enoyl-CoA hydratase is formally analogous to the fumarase reaction in the citric acid cycle, in which water adds across an α− β double bond. The hydration of enoyl-CoA is, in fact, the prelude to the second oxidation reaction, i.e., step 3.
Third step: Oxidation of β-hydroxyacyl-CoA.
In this step, the L-β hydroxyacyl-CoA is dehydrogenated (or oxidized) to form β-ketoacyl-CoA by the action of an enzyme, β-hydroxyacyl-CoA dehydrogenase, which is absolutely specific for the L-stereoisomer of the hydroxyacyl substrate. NAD+ is the electron acceptor in this reaction and the NADH, thus formed, donates its electrons to NADH dehydrogenase, an electron carrier of the respiratory chain. Three ATP molecules are generated from ADP per pair of electron passing from NADH to O2 via respiratory chain.
R–CH2–CH=CH–C–S–CoA + NAD+
∆ G´ = +3.75 kcal/mole
R–CH2–CO–CH2–C–S–CoA + O
β–Ketoacyl-CoA
NADH + H+ OH H
O
The reaction catalyzed by β-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid. Thus, we see that the first three reactions in each round of fatty acid oxidation closely resemble the last steps in the citric acid cycle:
Acetyl-CoA ⎯→ Enoyl-CoA ⎯→ Hydroxyacyl-CoA ⎯→ Ketoacyl-CoA
Succinate ⎯⎯→ Fumerate ⎯⎯→ Matate ⎯⎯⎯→ Oxaloacetate
The net result of the first three reaction is the oxidation of methylene group at β(or C-3) position to keep group of the substrate, acyl-CoA.
Fourth step → Thiolysis or Thioclastic scission
Thiolysis is the splitting by a thiol (–SH) group, aided by enzymatic catalysis. This final step brings about the cleavage of β-ketoacyl-CoA by the thiol group of a second mole of CoA, which yields acetyl-CoA, and an aryl-CoA, shortened by two carbon atoms. This thiolytlic cleavage is catalyzed by the enzyme, acyl-CoA acetyltransferase, which also have a broad specificity. This enzyme is more commonly called β-ketothiolase or simply thiolase.
R–CH2–CO–CH2–C–S–CoA + CoA–SH
∆ G´ = –6.65 kcal/mole coenzyme A
R–CH2–CO–S–CoA + CH3–C–CoA O
(n–2 carbons) O
β-ketoacyl-CoA (n carbons) Aceyl - CoA Acetyl - CoA
Although the overall reaction is reversible, the equilibrium position is greatly in the direction of cleavage.
As to the mechanism of thiolase action, the enzyme protein has a reactive thiol (–SH) group on a cysteinyl residue that is involved in the following series of reaction.
R–CH2–CO–CH2–C–S–CoA + Enz–SH R–CH2–CO–S–Enz +
β-ketoacyl-CoA Acetyl - CoA
O
Thiolase Acyl S-Enz
CH3–C–S–CoA O
R–CH2–CO–S–Enz + CoA–SH R–CH2–CO–S– CoA + Enz–SH Acyl-CoA
The shortening of a fatty acyl-CoA derivative by two carbon atoms can be represented by the equation:
R–CH2–CH2–CH2–CO–S–CoA + FAD + NAD + CoA–SH ⎯⎯⎯⎯→
R–CH2–CO–S–CoA + CH3–CO–S–CoA + FADH2 + NADH + H+
The shortened acyl-CoA then undergoes another cycle of oxidation, starting with the reaction catalyzed by acyl-CoA dehydrogenase β-ketothiolase, hydroxyacyl dehydrogenase and enoyl-CoA hydratase all have broad specificity with respect to the length of the acyl group. Thus, by repeated turns of the cycle, a fatty acid is degraded to acetyl-CoA molecules with one being produced every turn until the last cycle, wherein two are produced. The β-oxidation of fatty acids is presented in a cyclic manner.
R–CH2–CH2–C–S–CoA Acyl-CoA
O
β α
FAD FADH2
Acyl-CoA dehydrogenase
trans R–CH=CH–C–S–CoA O R–C–S–CoA
O CH3–C–S–CoA
O
Enoyl-CoA hydratase
H2O
R-CHOH–CH2–C–S–CoA O Thiolase
CoA–SH (Acetyl-CoA)
R–C–CH2–C–S CoA O O
(β-Ketoacyl-CoA)
NADH + H+ NAD+
β-Hydroxyacyl-CoA dehydrogenase
β-Enoyl-CoA
β-Hydrooxyacyl-CoA
The β-oxidation cycle for fatty acid
The β-oxidation system is found in all organisms. However, in bacteria grown in the absence of fatty acids, the β-oxidative system is practically absent but is readily induced by the presence of fatty acids in the growth medium. The bacterial β-oxidation system is completely soluble and hence is not membrane bound. Curiously, in germinating seeds possessing high lipid content, the β-oxidation system is exclusively located in microbodies called glyoxysomes, but in seeds with low lipid content, the enzymes are seen within the mitochondria.
When a fatty acid pass through a β-oxidation sequence, one molecule of acetyl-CoA, two pairs of electron, and four protons (H+) are removed from the long-chain fatty acyl-CoA, shortening it by two carbon atoms. The fatty acid is repeatedly β-oxidated for its complete breakdown to acetyl-CoA, electrons, and protons.
Yield of ATP during oxidation of one Molecule of Palmitoyl-CoA to CO2 and H2O
Enzyme catalyzing the oxidation step
Number of NADH or FADH2 formed
Number of ATP ultimately formed*
Acyl-CoA dehydrogenase 7FADH2 10.5
β-hydroxyacyl-CoA dehydrogenase
7NADH 17.5
Isocitrate dehydrogenase 8NADH 20
α-ketoglutarate dehydrogenase 8NADH 20
Succinyl-CoA synthetase GTP 8
Succinate dehydrogenase 8FADH2 12
Malate dehydrogenase 8NADH 20
Total 108
*These calculations assume that mitochondrial oxidative phosphorylation produces 1.5 ATP per FADH2 oxidized and 2.5 ATP per NADH oxidized.GTP produced directly in this step yields ATP in the reaction catalyzed by nucleoside diphosphate kinase.